Linear conductance switch



March 18, 1969 J NORDAHL ET AL LINEAR CONDUCTANCE SWITCH Filed March 6, 1967 5 2L; SUM m M 0 e w MW m m a 9 m m 4 ma 0 5 c uMN w M \m W. .1 a w w l n M m M n M m United States Patent 3,434,041 LINEAR CONDUCTANCE SWITCH John G. Nordahl, Lexington, and John Kaczorowski, Jr., Marshfield, Mass., assignors to Weston Instruments, Inc., Newark, N.J., a corporation of Delaware Filed Mar. 6, 1967, Ser. No. 620,935 US. Cl. 323-74 Int. Cl. H02p 13/16 14 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a linear conductance switch, and more particularly to a high precision linear conducduct ance switch which is highly accurate and relatively simple in construction.

Conductance switches can be used in electrical and electronic circuits to assist in controlling or adjusting the operation thereof. For example, such a switch may be connected to insert various values of conductance in series with a variable current source and a load, so that a constant voltage is developed across the inserted conductance for a particular setting of the current source and the conductance switch, whereby the current source can be adjusted if the voltage deviates from the constant voltage.

Where the inserted conductance must be precise, highly linear and accurate, the switch arrangement should meet certain requirements. It should be linearly variable in unit steps of one mho from zero mho to some convenient number, such as ten mhos. To insure long life for the contacts of the conductance switch, its wiper arm or arms should carry and switch as little current as possible between successive steps. The contact resistance of the switch contacts must not unduly affect the voltage developed across the conductance switch for any particular setting. In this regard, any error in the conductance occuring as a result of contact resistance should be as constant as possible over the entire range, without resorting to trimming or adjusting of the resistance elements. The conductance switch should be relatively simple in construction, in that it should use as few resistance elements as possible and each resistance element should be efiiciently used.

Heretofore, all of the above requirements have not been found in a single prior art conductance switch. One prior art conductance switch employs a standard single pole, twelve position switch, with ten resistors ranging in value from one ohm to 0.10 ohm to provide unit steps of conductance. However, the resistors are used very inefficiently in that each resistor is only used once. Also, the wiper arm of this prior art conductance switch conducts and switches relatively high currents up to ten amperes, and the conductance error varies from 0.1% to 1.0% (assuming that low resistance contacts on the order of 1 milliohm are used.) Other prior art conductance switches use either a three pole switch with four resistors in a binary scheme, or a two pole switch with five resistors in a modified binary scheme; however, both of these switches have some of the deficiencie noted above for the first mentioned prior art switch. Another conductance switch uses a single pole switch with ten equal- 3,434,041 Patented Mar. 18, 1969 valued resistors, and ten wiper arms movable through twenty-four positions. While this latter conductance switch conducts and switches relatively low current and has a relatively constant 10w error due to contact resistance, it is obvious that this switch is relatively complicated and cumbersome.

Accordingly, it is a general object of this invention to provide a linear conductance switch which overcomes the above-described problems and deficiencies associated with prior art conductance switches, and which has all of the requirements of a precise and highly accurate conductance switch.

Another object is to provide a high precision, linear conductance switch which is highly accurate and relatively simple in construction.

Another object is to provide a conductance switch of the type described, which is linearly variable in unit steps of conductance from zero mho to some convenient number of mhos, such as ten mhos.

Another object is to provide a conductance switch of the type described in which relatively low currents are carried and switched by its wiper arms.

Another object is to provide a conductance switch of the type described in which the contact resistance does not adversely affect the conductance between the terminals of the switch at any particular setting. A related object is to provide such a conductance switch in which the contact resistance error is relatively low and is within a relatively narrow range.

Another object is to provide a conductance switch of the type described, which is relatively simple in construction and uses a minimum number of resistance elements to provide unit steps of conductance from zero mho to ten mhos, while efiiciently using all of the resistance elements.

Broadly considered, a high precision linear conductance switch, according to this invention, includes a plurality of resistance elements each having one of its terminals connected to a first terminal. The resistance elements are arranged into two groups, each resistance element of the first group having a different value and having a corresponding resistance element in the other group. A plurality of wiper elements are connected to a second terminal. At a particular setting of the conductance switch, certain of the wiper elements electrically connect prescribed resistance elements in parallel relationship to produce a desired conductance value between the terminals.

In order that the manner in which the foregoing and other objects are attained in accordance with the invention can be understood in detail, particularly advantageous embodiments thereof will be described with reference to the accompanying drawings, which form a part of this specification, wherein:

FIG. 1 is a schematic diagram of a linear conductance switch according to the invention;

FIG. 2 is a block diagram of a circuit illustrating one use for the conductance switch of the invention; and

FIG. 3 is a schematic representation of a rotary-type conductance switch according to the invention.

Referring to FIG. 1, a conductance switch, according to the principles of the invention, is generally designated at '10, and includes an output terminal 11 and an input terminal 12 to which is connected conductor 13. One lead of each of a plurality of resistance elements is connected to conductor lead 13, the elements being divided into two groups 14 and 1 6. Group 14- includes three resistance elements 17, 118, and 19, having resistance values respectively, of one ohm, /2 ohm, and /3 ohm. Group 16 includes three resistance elements 21, 22, and 23 having resistance values respectively of one ohm, /2 ohm, and /3 ohm.

A first set of switch contacts 26 is associated with resistance element group 14, and a second set of switch contacts 27 is associated with resistance element group 16, the distance between adjacent contacts being equal. Contact set 26 includes fixed contacts 1'-1'1' and a contact set 27 includes fixed contacts 1"11".

One lead of each of resistance elements 17, .18 and 19 is connected to conductor 13. The other lead of resistance element 17 is directly connected to contact 1 of contact set 26, and, via a conductor 31 is connected to contacts 4" and 7". The other lead of resistance element 18 is directly connected to contact 3 and, via a conductor 32 to contacts 5' and 9'. The other lead of resistance element '19 is directly connected to contact 7' and, via a conductor 33 to contact '10.

One lead of each of resistance elements 21, 22 and 23 is connected to conductor 13. The other lead of resistance element 21 is directly connected to contact 2" and, via a conductor 34 to contacts 6 and 8'. The other lead of resistance element 22 is directly connected to contact 5" and via a conductor 36 to contact 9", and the other lea-d of resistance element 23 is directly connected to contact 8".

A plurality of wiper elements, arranged into two groups 38 and 39, are connected to output terminal 11. Group 38 includes two wiper elements 41 and 42, and group 39 includes three wiper elements 43, 44, and 45, all of the wiper elements being mechanically coupled together to be movable as a unit. When the wiper elements engage particular contacts, certain resistance elements are connected in parallel between terminals 11 and 12, and have an elfective conductance value corresponding to the number of the contacts so engaged by contacts 41 and 43. For example, with the wiper elements positioned as shown in FIG. 1, wiper elements 41 and 43 respectively engage contacts 5' and 5", wiper element 42 engages contact 4', wiper element 44 engages contact 4", and wiper element 45 engages contact 3". When the wiper elements are so positioned, 5 mhos of conductance are present between input terminal 11 and output terminal 12, resistance elements 17, 18 and 22 being connected in parallel circuit relationship between terminals 11 and 12 and presenting an effective resistance of 0.2 ohm and an effective conductance of 5 mhos.

In FIG. 2, there is shown a circuit illustrating one use for a conductance switch in accordance with the invention. The circuit includes a variable current source 46 connected in series with conductance switch 10 and a pair of terminals 47 and 48, an ammeter 49 to be calibrated being connectable to these terminals. In response to a particular current generated by current source 46, one volt is to be maintained across conductance switch 10, a voltmeter 50 being connected across conductance switch 10 to sense this voltage.

Since conductance (G) is the reciprocal of resistance (R), the current (I) is related to the voltage (E) across conducatnce switch 10 in the following manner:

It is convenient to have the dial (FIG. 3) of conductance switch .10 marked with indicia of unit steps of mhos from mho to 10 mhos. Thus, if meter 49 is to be calibrated for one ampere, the dial of the conductance switch is set to the one m'ho position. Substituting values into the above equation 1 ampere=1 volt 1 mho If voltmeter 50 does not read exactly one volt, current source 46 is adjusted until the voltmeter reads exactly one volt, thereby indicating that the current source is producing one ampere of current. Meter 49 can then be calibrated to read one ampere.

As another example and referring to FIG. 1, meter 49 is to be calibrated for amperes. The dial of the conductance switch is positioned at the 5 mho setting so that 5 mhos of conductance are presented between terminals 1.1 and .12, wiper elements 41-45 respectively engaging contacts 5', 4', 5", 4" and 3". Since exactly one volt is to be maintained across terminals 11 and 12 of conductance switch 10, exactly 5 amperes of current must be generated by current source 46. Substituting values into the above equation again:

5 amperes=1 volt 5 mhos Assuming that each contact of contact sets 26 and 27 has a resistance of one milliohm, then the error in conductance bet-ween input terminal '12 and output terminal 11 varies in a very narrow range from 0.10% (for the one mho setting) to 0.26% (for the ten mho setting). The following chart is a compilation of (1) the mhos (G) at a particular setting, (2) the effective resistance (R) at the particular setting, (3) the resistance elements connected in parallel at a particular setting, and (4) the percent error in actual conductance due to the contact resistance of the contacts.

In calculating the percent error, 1 milliohm is added to the resistance value of each resistance element connected in parallel for a particular dial setting, and then the eifective conductance is calculated for the particular resistance elements which comprise the conductance at that setting. The actual value of conductance is subtracted from the ideal value, and the difference is calculated as a percentage of the ideal conductance. From the above chart, it will be seen that the conductance error is very low and has a very narrow range for the various settings of the conductance switch.

From the foregoing, it will be appreciated that a physical embodiment (one of which will be described below) of the conductance switch of this invention is relatively simple in construction, in that only a total of six resistance elements are used and only three diiferent values of resistance are needed when the switch has ten settings of conductance. The conductance switch has all of the requirements that a linear, high precision and accurate conductance switch must have in order not to adversely affect the circuit that it is employed in, but has none of the disadvantages associated with prior art conductance switches. More particularly, the conductance switch is linearly variable in unit steps of one mho from zero mho to ten mhos. In tests performed on an actual embodiment of the conductance switch of the invention, only a maximum of three amperes of current were switched by any one of the switch contacts, and relatively low currents on the order of five or six amperes were carried by the wiper elements. Eificient use is made of each resistance element, each resistance element being used at least three times. And, when the switch contacts each have a resistance of one milliohm, the actual conductance varied from the ideal conductance over a very narrow range.

Because skilled persons in the electrical and electronics arts are accustomed to operating rotary-type devices (as opposed to sliding devices), the conductance switch of this invention may conveniently be embodied in a rotarytype device, such as the two-pole rotary device schematically shown in FIG. 3. A first pole 51 comprises a wafer 52 having formed thereon in a circular array the contacts of contact set 26, each contact being spaced from its two adjacent contacts by 30. Resistance element 17 is connected to conductor 13 and contact 1, resistance element 18 to conductor 13 and contact 3', and resistance element 19 to conductor 13 and contact 7'. A second pole 53 comprises a wafer 54 having formed thereon in a circular array the contacts of contact set 27, each contact being spaced from its two adjacent contacts by 30. Resistance element 21 is connected to conductor 13 and contact 2", resistance element 22 to conductor 13 and contact 5", and resistance element 23 to conductor 13 and contact 8". In the interest of clarity, the connections of the resistance elements to the other contacts via conductors 31-34 and 36 have not been shown in FIG. 3, but are understood to be the same as shown in FIG. 1.

A first rotor 56 has mounted thereon wiper elements 41 and 42, the wiper element being spaced apart so that each engages a contact on wafer 51 for a particular mho setting of the switch. A second rotor 57 has formed thereon wiper elements 43-45, the wiper elements being spaced apart so that each engages a contact on wafer 54 for a particular mho setting. Rotors 56 and 57 are ganged together via a rigid shaft 58 for rotation together by a control knob 59. Knob 59 also controls the movement of a dial indicator 6 over a dial 62, dial indicator 61 being rotatable in unit steps of one mho from a zero mho setting through a ten mho setting. As depicted in FIG. 3, dial indicator 61 is disposed on the five mho setting, indicating that five mhos of conductance are present between terminals 11 and 12.

The rotary-type embodiment depicted in FIG. 3 may be custom built; or it can be constructed from a conventional two-pole twelve position switch device, in which each rotor has a plurality of wiper elements, the wiper elements that are not required being removed and the resistance elements being connected as described with respect to FIG. 1. It will be apparent to those skilled in the art that the conductance switch of this invention can be embodied in a variety of other physical forms, in addition to that described with respect to FIG. 3. For example, instead of using two wafers 51 and 53, a single wafer may be employed having the respective contact sets formed on opposite sides thereof. The conductive switch may also be embodied in a slide-type device, which could look very much like the schematic diagram of FIG. 1. Also, the resistance elements may be any resistive device, such as carbon resistors, wire-Wound resistors, printed circuit-type resistors, integrated-circuit resistors, and the like.

It will be appreciated, that in a broad sense, the conductance switch of this invention provides values of conductance between terminals 11 and 12 from zero to n mhos in unit steps, and uses two groups of resistance elements, wherein each resistance element of the first group has a different resistance value and has a corresponding resistance element in the other group, the other group having an extra resistance element(s) depending on the value of n. For the scheme described in FIG. 1, n equals ten, and two resistance element groups 14 and 16 are employed wherein each group has three resistance elements. The scheme, where n equals ten, is very convenient since it permits an operation to switch conductance values into and out of a circuit in decade fashion. If n equals six, then group 14 is made up of resistive elements 17 and 18, and group 16 is made up of resistance elements 21 and 22, as will be apparent by referring to the chart. It will also be apparent from referring to the chart that if the scheme is expounded beyond n equals ten then the error range will widen somewhat. Those skilled in the art, having the teaching of this invention before them, will be able to devise other schemes, if the need arises to do so, where n equals some other number than six or ten.

What is claimed is:

1. A conductance switch, having a first terminal and a second terminal, for providing preselected values of conductance between the terminals, comprising: a plurality of resistance elements each having one of its leads electrically connected to one of the terminals, said resistance elements being arranged into two groups, said resistance elements of one of said groups each having a different resistance value and each having a corresponding resistance element in said other group; a plurality of fixed electrical contacts, the other lead of each resistance element being electrically connected to at least one of said contacts; and a plurality of moveable wiper elements electrically connected to the other terminal, so that at a particular conductance setting certain of said resistance elements are connected in parallel across the first and second terminals via certain ones of said fixed contacts which said Wiper elements engage.

2. The conductance switch according to claim 1, wherein said plurality of wiper elements are arranged into two groups, one of said groups of wiper elements having two such wiper elements and being arranged to engage said fixed contacts connected to said resistance elements of said one group of resistance elements, the other of said groups of wiper elements having three such elements and being arranged to engage said fixed contacts connected to said resistance elements of said other group of resistance elements.

3. The conductance switch according to claim 1, wherein said conductance switch provides values of conductance between the first and second terminals for 0 to n mhos in units steps.

4. The conductance switch according to claim 3 in which n equals six and wherein each group of resistance elements comprises: two resistance elements having resistance values respectively of one ohm and one-half ohm.

5. The conductance switch according to claim 3 in which n equals ten, and wherein each group of resistance elements comprises three resistance elements having resistance values respectively of one ohm, one-half ohm, and one-third ohm.

6. A conductance switch, having a first terminal and a second terminal, for providing values of conductance from zero to n mhos in unit conductance steps, comprising: a plurality of resistance elements, each having one of its leads electrically connected to one of the terminals and arranged into two groups, said resistance elements of one of said groups each having a ditferent resistance value and each having a corresponding resistance element in the other of said groups; a first set of switch contacts asso ciated wtih said first group of resistance elements; a second set of switch contacts associated with said second group of resistance elements; a plurality of wiper elements electrically connected to the other terminal and arranged into two groups, said first group having two of said wiper elements which are adapted to engage predetermined contacts of said first contact set for each eonductance setting, said second group having three of said wiper elements which are adapted to engage predetermined contacts of said second contact set for each conductance setting, said wiper elements being moveable as a unit so that for a particular conductance setting certain of said resistance elements are connected in parallel between the first and the second terminals.

7. The conductance switch according to claim 6 in which n equals ten, wherein each of said groups of resistance elements comprises three resistance elements having resistance values respectively of one ohm, one-half ohm, and one-third ohm.

8. A conductance switching having a first and a second terminal, for providing values of conductance from 0 to n mhos in unit steps, comprising: wafer means having formed thereon first and second sets of fixed electrical contacts, each contact of each of said sets of contacts being equally spaced from adjacent ones of said contacts; a plurality of resistance elements each having one of its leads electrically connected to one of the terminals and arranged into two groups, each of said resistance elements of said first groups having a different resistance value and each having a corresponding resistance element in said second group, the other lead of each of said resistance elements being connected to at least one of said contacts; rotor means; and a plurality of wiper elements electrically connected to the other terminal and disposed on said rotor means, so that for a particular conductance setting of said conductance switch, each of said wiper elements engage a respective one of said fixed contacts to connect certain of said resistance elements in parallel across the first and second terminals.

9. The conductance switch according to claim 8 wherein said Wafer means comprises: a first wa'fer having said first set of contacts formed thereon, and a second wafer and having said second set of contacts formed thereon.

10. The conductance switch according to claim 9 wherein each of said sets of contacts includes: twelve contacts disposed in a circular array on its corresponding wafer, each contact being spaced 30 from each of its two adjacent contacts.

11. The conductance switch according to claim 9, wherein the rotor means comprises: a first rotor carrying two wiper arms disposed to engage two adjacent contacts on said first wafer for each setting of said conductance switch; and a second rotor carrying three wiper arms disposed to engage three adjacent contacts on said second water for each setting of said conductive switch, said first and second rotors being moveable as a unit so that at a particular setting of said conductance switch said wiper arms of said respective rotors engage certain of said contacts, whereby certain of said resistance elements are connected in parallel to produce a conductance value between the first and second terminals corresponding to the particular setting.

12. The conductance switch according to claim 11, wherein each of said sets of contact elements includes: twelve contacts disposed in a circular array on its corresponding wafer, each contact being spaced 30 from each of its two adjacent contacts.

13. The conductance switch according to claim 12 further comprising: control knob means for moving said first and second rotors as a unit to selectively connect certain of said resistance elements in parallel between the first and second terminals for a particular setting, and for indicating the conductance value.

14. The conductance switch according to claim 13 in which n equals ten, and wherein each of said groups of resistance elements comprises three resistance elements having resistance values respectively of one ohm, one-half ohm, and one-third ohm.

References Cited UNITED STATES PATENTS 2,786,122 3/1956 Strain 323-74 X 2,938,156 5/1960 Smith 323-74 2,951,200 8/1960 Critchlow 32374 X 3,252,080 5/ 1966 Newbold et a1 323-74 JOHN F. COUCH, Primary Examiner.

G. GOLDBERG, Assistant Examiner.

U.S. C1. X.R.

Disclaimer and Dedication 3,434,041.J0hn G. Nomlahl, Lexington, and John Kaczom'wski, J12, Marshfield, Mass. LINEAR CONDUCTANCE SWITCH. Patent dated Mar. 18, 1969. Disclaimer and dedication filed Mar. 17, 1971, by the assignee, Weston lnstmmnts, Inc. Hereby enters this disclaimer to the remaining term of said patent and dedicates said patent to the Public.

[Ofiicial Gazette April 27, 1.971.] 

